AVS 72 Session NS-MoA: Frontiers in Nanoscale Electron, Ion, and Scanning Probes II

Surface phonon polaritons (SPhPs) are hybrid electromagnetic modes that arise at dielectric-vacuum interfaces from the coupling of mid-infrared electromagnetic radiation with optical phonons in polar dielectric materials. Because thermal radiation near room temperature is predominantly in the mid-infrared, SPhPs play a significant role in radiative heat transfer (RHT), particularly at the nanoscale. Indeed, theoretical predictions made several decades ago showed that RHT can exceed the blackbody limit by several orders of magnitude when two surfaces are separated by nanometric gaps, a phenomenon known as near-field radiative heat transfer (NFRHT). Experimental demonstrations over the past decade have further increased interest in understanding and controlling NFRHT.

In this work, I will describe how state-of-the-art experiments have revealed new insights into SPhP-mediated NFRHT and its dependence on thickness and temperature. In the first part, I will present our measurements of thickness-dependent NFRHT between planar magnesium fluoride nanofilms, performed using microfabricated devices and a custom-developed nanopositioner [1]. Our results show an 800-fold enhancement in RHT above the blackbody limit and demonstrate that nanofilms can be as effective as bulk materials when the gap between the two surfaces is smaller than the film thickness.

In the second part, I will briefly discuss our work on NFRHT between a silica sphere and a planar silica surface from 77 to 300 K [2]. Our results reveal that cryogenic NFRHT has strong contributions from SPhPs and does not follow the temperature dependence expected from far-field radiative thermal conductance. The microfabricated devices and techniques developed in this work enable advances in nanoscale RHT, with relevance to applications in thermal management, energy conversion, and photonic cooling technologies.

1. R. Mittapally, J. W. Lim, P. Reddy, E. Meyhofer, B. Song. Quantifying the effect of nanofilms on near-field radiative heat transfer. ACS Photonics, 10 (8), 2474–2480 (2023)
2. S. Yan, Y. Luan, J. W. Lim, R. Mittapally, A. Reihani, Z. Wang, Y. Tsurimaki, S. Fan, P. Reddy, E. Meyhofer. Surface phonon polariton-mediated near-field radiative heat transfer at cryogenic temperatures. Physical Review Letters, 131 (19), 196302 (2023)

Twisting and stacking two-dimensional materials provides an innovative method for designing materials with novel properties. Moiré superlattices of 2D van der Waals (vdW) materials can host strong correlations that lead to unconventional and emergent phenomena, such as superconductivity and magnetism. Recently, 3D free-standing perovskite oxide membranes have been prepared by employing a sacrificial layer. Their moiré superlattices are shown to host topological polar vortices and new electronic states. In this study, we investigate using analytical transmission electron microscopy two free-standing SrTiO₃ membranes stacked at a 45° twist angle. The STO membranes are synthesized by epitaxial growth on a SrO sacrificial layer, utilizing a hybrid MBE method, followed by dissolution of the SrO layer. The 45° twist angle between the top and bottom membranes produces an 8-fold rotational symmetry at the interface, when viewed using HAADF-STEM along the plan-view orientation. This symmetry, forbidden in periodic crystals, demonstrates the ordered but incommensurate quasi-periodic structure at the interface. Ti-L_2,3 and O-K EELS mapping are also performed along the twisted interface stack. Significant changes are observed in the Ti-L_2,3 edge fine structure at the interface as compared to that of the bulk STO spectra. The O-K EELS edge at the interface is also dissimilar to that of the bulk. The changes in the Ti-L_2,3 and O-K EELS spectra at the twisted membrane interface arise from novel electronic states present due to the quasi-periodic moiré structure of the interface.

Supported metal nanoparticles are central to heterogeneous catalysis, yet their evolution is traditionally modeled assuming static, inert supports. In situ TEM observations show that dynamic substrate transformations reshape nanoparticle behavior through interfacial energy landscapes. However, the mechanisms by which propagating phase boundaries couple to particle motion remain unexplored. Quantifying such coupled dynamics requires extracting quantitative information from high-resolution time-series microscopy data, a technically demanding task that until now has limited our ability to build predictive models at the nanoscale.

We developed an unsupervised convolutional neural network framework for automated segmentation and real-time tracking of individual nanoparticles in high-resolution in situ TEM image sequences. The framework extracts size, position, and trajectory data for hundreds of particles simultaneously with minimal human intervention. By pairing automated image analysis with time-resolved diffraction, we establish direct temporal correlations between substrate transformation and particle motion.

Applying this framework to Pt nanoparticles on crystallizing AlOₓ demonstrates that propagating amorphous-to-crystalline transformation fronts actively drag supported particles over long distances through interfacial energy gradients. Temporal correlation between SAED-detected crystallization onset and rapid particle migration, combined with 4D-STEM virtual crystallinity maps, directly links substrate dynamics to particle transport. Phase-field simulations confirm that asymmetric particle-substrate interfacial energy at the advancing front acts through curvature gradients to redistribute atomic mass and sustain long-range drag.

The results establish a general principle: any propagating solid-state phase boundary generating surface-energy contrast can deterministically drive nanoparticle organization. These mechanistic insights enable rational design of catalyst supports through deliberate engineering of substrate crystallization sequences or separated crystalline regions, offering pathways to suppress unwanted sintering or direct particle assembly for enhanced catalytic performance.

New materials exhibiting robust polarization reversal and CMOS-compatible processing (e.g. alloys based on AlN, ZnO, and HfO₂) provide new opportunities for integrated ferroelectrics. To understand the mechanisms that control and induce switchable states in these materials, including the influence of microstructure and electrode interfaces, we employ a variety of transmission electron microscopy (TEM) and spectroscopy techniques to study and quantify structure from micrometer to sub-ångstrom length scales. In particular, scanning TEM (STEM) imaging is used to determine local atomic structure that is subsequently utilized to calculate partial pair distribution functions and local spontaneous polarization. The atomic- to meso-scale structural and chemical insights obtained through STEM-based microscopy, including those associated with interfaces, grain boundaries, domain boundaries, and their interactions, are invaluable for understanding polarization phenomena in these emerging classes of ferroelectrics. Furthermore, in situ biasing experiments provide insight into domain nucleation and domain wall propagation, both of which remain topics of active investigation and exhibit behavior distinct from that of more traditional perovskite ferroelectrics.

Perovskite oxide heterostructures provide a versatile platform for engineering emergent electronic and magnetic phenomena through atomic-scale control of interfaces. In particular, SrTiO₃/LaMnO₃ (STO/LMO) systems exhibit enhanced interfacial magnetism arising from structural disorder; however, the influence of local termination chemistry and defect-driven heterogeneity on electronic structure remains poorly understood. This is because studying these interfaces across a few unit cells is challenging with conventional methods, as optical probes tend to be around 1 μm in diameter. This makes it’s impossible to study an individual interface of a vertically stacked transition metal oxide from a cross-sectional sample as the optical probe collects an averaged signal from each layer. In this talk, electron probes are used to overcome the diffraction-limit set by optical probes. This is done by converging electrons to a focused beam to collect spectroscopic signal from an oxide-interface with electron energy loss spectroscopy (EELS). A monochromated aberration corrected scanning transmission electron microscopy (MAC-STEM) is used to couple unprecedented energy resolution and spatial resolution to do optical and vibrational spectroscopy at atomic resolution. When the EELS aperture is in conventional settings, where the bright field disc is perfectly aligned with the entrance aperture, the EELS signal is delocalized and dominated by optical selection rules. Accordingly, the bright field disc is electrostatically shifted by the projector lens so that the dark field disc is highlighted, which describes a technique known as off-axis EELS. We use this to create a localized signal dominated by impact scattering so that the long-range contributions are suppressed, and band edges can be resolved without plasmon interference. Ultimately, we aim to introduce a novel technique to determine the structure-property relationship of complex oxides to bring oxide-electronics to new heights.

High quality AFM imaging is time-consuming process.With the current emerging deep learning concept, can low-resolution AFM images be improved by such technique? In this study (1) a model sample was imaged under standard ambient scanning conditions. Both traditional image enhancement methods and deep learning models were applied to improve image resolution and were benchmarked in terms of fidelity, quality, and expert evaluation by AFM experts. The deep learning models outperform the traditional methods, yielding superior results. Furthermore, common AFM artifacts such as streaking, present in the high-resolution ground truth images, were only partially attenuated by traditional methods but were eliminated by the deep learning models. This work demonstrates that deep learning models are superior for super-resolution tasks and enable a significant reduction in AFM measurement time, as low-pixel-resolution AFM images can be enhanced in both resolution and fidelity through deep learning.

(1) Beilstein J. Nanotechnol. 2025, 16, 1129–1140.

Metalloenzymes, enzymes containing a metal co-factor, are of great interest in green energy production. Although their structure is well known, their high efficiencies remain poorly understood, as calculating reorganization energies, i.e. the coupling of electronic and vibronic degrees of freedom, is challenging for large systems. As a result, modelling requires multiple approximations that need to be validated by experiments.

We use electrostatic force microscopy (e-EFM) to measure energy levels, single-electron tunneling rates, reorganization energies, and transitions between vibronic states with single molecule resolution. In e-EFM, single electron charge transfer between a back electrode and the sample lead to a change in cantilever frequency shift and dissipation. These are measured at cryogenic temperatures with well-established frequency modulated atomic force microscopy techniques. By changing the bias between the tip and the back electrode the electron energy levels of the system can directly be quantified. The ratio of the dissipation rate and frequency shift is proportional to the charge transfer rate; the energy dependence of this rate allows the degeneracy of the state to be determined. Importantly, we have shown that force detected Franck-Condon blockade can be used to directly measure molecular vibrations and reorganization energies at the single molecule level.

We have adapted a system developed by Abad et al., where the metal center of a metalloenzyme is electrically connected to a gold back electrode via a functionalized 1.6 nm gold nanoparticle (AuNP), which is covalently bound to a conductive self-assembled monolayer (SAM). This sample architecture ensures an electrical contact between the gold back electrode and the metalloenzyme. Preliminary e-EFM measurements of the AuNP–SAM system alone confirm high SAM conductance. Bias spectroscopy reveals highly localized sharp Coulomb blockade features in both frequency shift and dissipation. We extract tunneling rates consistent with electrochemical measurements from these sites. This shows that the SAM-AuNP is not the charge transfer rate-limiting step, thus validating the enzyme-AuNP-SAM-electrode system. On-going single-electron spectroscopy on metalloenzyme-functionalized AuNP-SAM-electrode devices will enable direct measurements of enzymatic electron transfer. This will allow spatial variations of the redox as well as reorganization energy to be measured and will provide experimental validation of theoretical electrochemical models.

1. T. Huff et. al., “Molecular tools for non-planar surface chemistry,” arXiv:2508.16798 [cond-mat.mtrl-sci] (2025), https://doi.org/10.48550/arXiv.2508.16798 (submitted for peer review).
2. T. McCallum, et. al., “(Hetero)adamantane synthesis: A triple alkylation reaction,” https://doi.org/10.26434/chemrxiv.15002728/v1 (submitted for peer review)
3. E. Barrera et. al., “Inverted-mode scanning tunneling microscopy for atomically precise fabrication,” arXiv:2512.24431 [cond-mat.mes-hall] (2025), https://doi.org/10.48550/arxiv.2512.24431 (submitted for peer review).

Control of destructive interference between neighboring electrons in frustrated hopping systems results in highly tunable electronic properties stemming from real-space topology. The kagome lattice has been a representative example of frustrated hopping system that hosts flat band accompanied by spatially localized electronic states. In real kagome materials, however, it is difficult to tune hopping energies independently, making it challenging to separate destructive-interference effects frommaterial-dependent structural and orbital effects. Here, we use atomic manipulation with a scanning tunneling microscope to build molecular kagome lattices. Carbon monoxide (CO) molecules are individually positioned on Cu(111) to confine surface states at the kagome lattice sites. By varying the lattice constant, we tuned the hopping energies and measured local density of states associated with flat band using scanning tunneling spectroscopy over various hopping ratio t_NN/t_NNN. A defect was introduced to probe the real-space response of the flat band states, which confirms the frustrated hopping model in molecular kagome lattices.